
1891.] The Litopterna. 691 
cusps into more or less distinct Vs of equal proportions, while 
the internal part of the crown is modified from transverse crests, 
such as are seen in the Toxodontia. The genera are as follows: 
I. Canines one-rooted. , 
? Canines; incisors. small and sepa- 
rated by diastemata ; Diatomicodon Amegh. 
Inferior molars with simple Vs ; Scalabrintherium Amegh. 
Vs of inferior molars with an external 
transverse branch; Oxyodontotherium Amegh. 
II. Canines above two-rooted. 
? Canines; external nares advancing 
to between premaxillaries ; Mesorhinus Amegh. 
External nares bounded by maxillaries 
in front ; Macrauchenia Ow. 
But little is known of these genera, excepting Macrauchenia. 
In this genus there are three subequal toes, both anteriorly and 
posteriorly, and progression was digitigrade. The nostrils were 
remarkable for their posterior position, being partly above the 
orbits. The latter are closed behind. The last premolar teeth 
only in both jaws are like the molars, and there is a short dias- 
tema in front of the small canin Incisors like those of a horse, 
without the cups. Dental formula: I. $; C.4; Pm. 4; M. $. 
The molars, as is often the case in the Argentine forms, soon 
wear so as to obscure the structure, and an internal cingulum 
encloses the spaces between the cross-crests as fosse. The 
absence of vertebrarterial canal from the cervical vertebre in 
this genus is a repetition of what occurs in the Camelide. Four 
species of Macrauchenia are described (Plate XVII). ; 
Macrauchenia patachonica Owen was an animal of about the | 
proportions of a mule. The posterior position of the external 
nostrils has suggested that the habits were aquatic, or that it 
possessed a short trunk like the tapir. It is a characteristic 
species of the Pliocene beds of Buenos Ayres. A smaller 
species, M. boliviensis Huxley, has been found in corresponding 
beds in Bolivia. Scaladrintherium paranense Brav. and orthii 
Amegh. are from Miocene beds of the Paraña. The same form- 
ation has produced the Mesorhinus pyramidatus Amegh., which 

